Advances in the Synthesis of Small Molecules as Hole Transport

Article Cite This: Acc. Chem. Res. 2018, 51, 869−880

pubs.acs.org/accounts

Advances in the Synthesis of Small Molecules as Hole Transport Materials for Lead Halide Perovskite Solar Cells Cristina Rodríguez-Seco,† Lydia Cabau,† Anton Vidal-Ferran,*,†,‡ and Emilio Palomares*,†,‡ †

Institute of Chemical Research of CataloniaThe Barcelona Institute of Science and Technology (ICIQ-BIST), Avda. Països Catalans 16, E-43007 Tarragona, Spain ‡ ICREA, Passeig Lluis Companys 23, E-08010 Barcelona, Spain S Supporting Information *

CONSPECTUS: Over hundreds of new organic semiconductor molecules have been synthesized as hole transport materials (HTMs) for perovskite solar cells. However, to date, the well-known N2,N2,N2′,N2′,N7,N7,N7′, octakis-(4methoxyphenyl)-9,9-spirobi-[9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine (spiro-OMeTAD) is still the best choice for the best perovskite device performance. Nevertheless, there is a consensus that spiro-OMeTAD by itself is not stable enough for long-term stable devices, and its market price makes its use in large-scale production costly. Novel synthetic routes for new HTMs have to be sought that can be carried out in fewer synthetic steps and can be easily scaled up for commercial purposes. On the one hand, synthetic chemists have taken, as a first approach, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the spiro-OMeTAD molecule as a reference to synthesize molecules with similar energy levels, although these HOMO and LUMO energy levels often have been measured indirectly in solution using cyclic voltammetry. On the other hand, the “spiro” chemical core has also been studied as a structural motif for novel HTMs. However, only a few molecules incorporated as HTMs in complete functional perovskite solar cells have been capable of matching the performance of the best-performing perovskite solar cells made using spiro-OMeTAD. In this Account, we describe the advances in the synthesis of HTMs that have been tested in perovskite solar cells. The comparison of solar cell efficiencies is of course very challenging because the solar cell preparation conditions may differ from laboratory to laboratory. To extract valuable information about the HTM molecular structure−device function relationship, we describe those examples that always have used spiro-OMeTAD as a control device and have always used identical experimental conditions (e.g., the use of the same chemical dopant for the HTM or the lack of it). The pioneering work was focused on well-understood organic semiconductor moieties such as arylamine, carbazole, and thiophene. Those chemical structures have been largely employed and studied as HTMs, for instance, in organic light-emitting devices. Interestingly, most research groups have reported the hole mobility values for their novel HTMs. However, only a few examples have been found that have measured the HOMO and LUMO energy levels using advanced spectroscopic techniques to determine these reference energy values directly. Moreover, it has been shown that those molecules, upon interacting with the perovskite layer, often have different HOMO and LUMO energies than the values estimated indirectly using solution-based electrochemical methods. Last but not least, porphyrins and phthalocyanines have also been synthesized as potential HTMs for perovskite solar cells. Their optical and physical properties, such as high absorption and good energy transfer capabilities, open new possibilities for HTMs in perovskite solar cells. and organic solar cells.4 Nevertheless, the device configuration in the most efficient perovskite solar cells still resembles that in solid-state DSSCs (Figure 1 left). Upon light irradiation, free carriers (electrons and holes) are photogenerated in the perovskite material and transported to the selective contacts. Whether movement of other charges related to the composition of the perovskite material (e.g., cations, anions, or protons) compasses this transport of carriers is still a matter of active scientific debate.5

1. INTRODUCTION During the last 5 years, perovskite solar cells have been the focus of intense research. Back in 2009, the record efficiency for these devices was 3.8%,1 but by 2016 the efficiency reached beyond 22%.2 The major difference was to change the liquid electrolyte employed to collect the electronic holes by a solid organic semiconductor. In fact, at the beginning, methylammonium lead iodide (MAPI) with the perovskite structure was considered as a “pigment” in dye-sensitized solar cells (DSSCs).3 More recently, researchers have discovered that MAPI and other hybrid perovskites behave as inorganic semiconductors (as, for instance, GaAs) with different operation than DSSCs © 2018 American Chemical Society

Received: November 29, 2017 Published: March 15, 2018 869

DOI: 10.1021/acs.accounts.7b00597 Acc. Chem. Res. 2018, 51, 869−880

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Accounts of Chemical Research

Figure 1. (left) Pictorial representation of the layers commonly found in perovskite solar cells. (right) Structure of spiro-OMeTAD.

On the one hand, the most used contact for the electrons is a thin layer of mesoporous TiO2 or a thin layer of fullerene derivatives.6 On the other hand, different organic molecules have been used as contacts for holes. To date, N2,N2,N2′,N2′,N7,N7,N7′,N7′octakis-(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′tetramine (spiro-OMeTAD) (Figure 1 right; also see the Supporting Information) has been the best molecule to achieve reproducible and highly efficient perovskite solar cells. In fact, spiro-OMeTAD was the molecule of choice for the best solidstate DSSCs with efficiencies close to 7.2%.7 Great effort has been devoted to optimizing the composition of the perovskite material to achieve greater efficiencies. In parallel, studies to understand the perovskite solar cell function and

Figure 2. Linear π-conjugated HTMs developed by Li and co-workers.

Figure 3. H101 hole transport material and its analogues. 870


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Figure 4. Different families of HTMs with different redox potentials bearing diaryl- and triarylamine moieties employed to study the effect of the HOMO energy level on the solar cell Voc.

its limitations are also currently ongoing.8 All of these results are helping to establish the first models for solar cell operation and will allow a design for new contacts to be established. In this Account, we analyze most of the work done on the synthesis of hole transport materials (HTMs) that are used as selective contacts. Despite the large number of attempts to discover new HTMs, spiro-OMeTAD is still the material of choice because of the high efficiencies achieved. Nonetheless, there is a scientific consensus that for long-term stable and cost-effective perovskite solar cells, other HTMs must be sought.

Mhaisalkar, Grimsdale, and co-workers.12 H101 (Figure 3) was among the very first “linear” HTMs that could match spiroOMeTAD. Solar cell efficiencies as high as 13% at 1 sun were reported. However, both the H101 and the spiro-OMeTAD were doped. Later, the 2,3-dihydrothieno-[3,4-b]-[1,4]-dioxine-5,7diyl)dimethanimine (EDOT) central core in EDOT-OMeTPA was substituted by furan (F101),13 thiophene (H111),14 and 2,2′bithiophene (H112) cores14 with almost identical results regarding solar cell performance. HTMs containing a butadiene fragment (see HTM1 and HTM2 in Figure 4) have also been employed to synthesize low-molecularweight HTMs.15 However, the Tg values were lower than that of spiro-OMeTAD (Tg = 120 °C), which compromises the device stability. An ultraviolet photon spectroscopy (UPS) study was carried out and confirmed that the values estimated from theoretical work differed significantly for the highest occupied molecular orbital (HOMO) energy. We consider this finding particularly interesting, as UPS studies are scarce in the perovskite/HTM scientific literature. Other authors focused their studies, using “linear” HTMs bearing 4-(bis-(4-substituted-aryl)-amino)phenyl groups, on the effect of redox potential differences, having in mind the possible differences in the HOMO energy alignment with the perovskite-like material’s valence band (VB). For instance, using HTMs that contain a buta-1,3-diyne core (see MeO-DATPA and Me2N-DATPA in Figure 4), Snaith, Robertson, and co-workers16 could control a wide range of oxidation potentials above and below the redox value measured for spiro-OMeTAD. The devices showed differences in open-circuit voltage (Voc) but because of the lower hole mobility values (measured using SCLC) compared with spiroOMeTAD, it is still difficult to draw a conclusion. Seok and coworkers17 prepared HTM materials having diarylaminopyrene motifs (see Py-A, Py-B, and Py-C in Figure 4). Again, no trend between the redox potential of the HTM and the Voc of the solar cell was observed. More complex structures have been developed as HTMs using symmetrically substituted cores incorporating 4′-(bis-(4-methoxyphenyl)-amino)-[1,1′-biphenyl]-4-yl substituents

2. SMALL ORGANIC MOLECULES AS ARYLAMINE-SUBSTITUTED HTMS The choice of known HTMs in organic semiconductors is endless. For example, Xiao, Li, and co-workers9 reported a series of linear π-conjugated HTMs having diaryl- and triarylamino groups. They found that increasing the length of the π-conjugated bridge on these structures led to lower hole mobility (measured using space-charge-limited current (SCLC)) and poorer performance, and although they have similar oxidation potentials as spiroOMeTAD, only the HTM 2TPA-2DP (Figure 2) had an efficiency close to that of the spiro-OMeTAD-based device used for comparison. Huang, Shen, and co-workers10 used azines and azomethines as molecular bridges. Those compounds showed lower lowest unoccupied molecular orbital (LUMO) energies compared with spiro-OMeTAD, and their efficiencies did not go beyond 7%. Moreover, the weakness of these compounds is their need to be protonated to show good hole mobility values as measured using SCLC. Docampo and co-workers also used azomethine moieties.11 The HTM EDOT-OMeTPA (Figure 3) can be efficiently synthesized by the formation of two CN bonds through condensation of the corresponding bisaldehyde with N1,N1-bis-(4methoxyphenyl)-benzene-1,4-diamine. EDOT-OMeTPA was designed to increase the glass transition temperature (Tg), thus increasing the device stability, and to lower the LUMO energy level in order to, a priori, improve the energy band alignment compared with the HTM H101 reported beforehand by 871


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Figure 5. Some examples of HTMs bearing triarylamine moieties and different central cores.

(see B-[BMPHDPH]2, DPEDOT-B-[BMPDP]2 and DPBTDB-[BMPDP]2 in Figure 5)18 and 4,4-disubstituted 4H-silolo[3,2-b:4,5-b′]dithiophene motifs (see structures PEH-1 and PEH-2 in Figure 5),19 among other cores (structures TPAMeOPh and FA-MeOPh in Figure 5).20 However, in all cases, despite the increase in the number of arylamine moieties, the Voc

was lower than 1 V, and no relation between their redox properties and the measured Voc was observed. Synthetic strategies to “mimic” the spiro motif have also been studied. Grimsdale, Mhaisalkar, and co-workers used a decorated triptycene core to synthesize HTMs with good thermal stability and appropriate redox properties (structures T101, T102, and 872


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than spiro-OMeTAD,22,23 (structures PST-1 and KTM-3 in Figure 5), with the exception of the HTMs developed by Chi, Chen, and co-workers24 (structures CW3, CW4, and CW5 in Figure 5), which show an efficiency of 16.56%. The use of triphenylamine-based HTMs without the use of dopants has also been explored by Yi et al.25 (structure Z1011 and related moieties in Figure 5). Interestingly, the use of chemical dopants had a negative effect on the performance of Z1011 in perovskite solar cells, in clear contrast to spiro-OMeTAD. In fact, the lack of dopants in Z1011 solar cells improves the long-term stability of these cells. Z1011 was designed to have a higher Tg, but the stilbenelike motifs in its structure undergo faster chemical oxidation in the presence of chemical dopants or oxygen. Last but not least, the best efficiencies observed with arylaminecontaining HTMs have been reported for the molecule FDT (Figure 5). The molecular design consists of the introduction of a [2,2′-bithiophene]-3,3′-diyl fragment/motif in the central spiro core with the rationale that the sulfur atoms of the bithiophene group will interact with the iodine present in the perovskite material and favor the hole transfer process. Moreover, the [2,2′bithiophene]-3,3′-diyl fragment/motif in the central spiro core does not noticeably change the HOMO energy level (−5.16 eV) with respect to that of the reference spiro-OMeTAD (−5.14 eV). Using chemical dopants, Nazeeruddin et al.26 reported efficiencies as high as 20.2% for materials processed from toluene instead of chlorobenzene.

Figure 6. Carbazole-based HTMs for efficient perovskite solar cells.

Figure 7. Structure of the hole transport material TPDI.

3. SMALL ORGANIC MOLECULES AS HTMS WITHOUT ARYLAMINE SUBSTITUENTS It was also of interest to use carbazole units as HTMs. For instance, Wang, Wu, and collaborators demonstrated an easy and

T103 in Figure 5).21 Nevertheless, despite the fact that the Voc matched the values measured for spiro-OMeTAD devices, the overall efficiencies were lower, mainly as a result of the lower photocurrent.21 Other examples also showed lower performance

Figure 8. Carbazole containing HTMs and luminescent decays using TCSPC. 873


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Figure 9. Examples of thiophene containing small semiconductor organic molecules.

straightforward synthesis of R01 (Figure 6).28 This molecule displayed an excellent hole mobility (2.05 × 10−4 cm2 V−1 s−1) measured using SCLC and an oxidation potential similar to that of spiro-OMeTAD. The solar cell efficiencies when these two HTMs (i.e., R01 and spiro-OMeTAD) were doped were quite similar (about 12% at 1 sun), but as an advantage, less dopant was used per mole of R01.27 Fused carbazoles, such as C12-carbazole described by Shrestha, Han, and co-workers,28 have also been tested. The reported devices derived from C12-carbazole also displayed better efficiencies than those from spiro-OMeTAD as a result of a better fill factor, although the Voc was lower despite the lower energy of the HOMO. The HTM C12-carbazole was seen by Yang and co-workers as an alternative to spiro-OMeTAD.29 In this case, the hole mobility (measured using SCLC) was outstanding, with reported values of (3−5) × 10−3 cm2 V−1 s−1, with a lower HOMO energy and good thermal stability. The conjugated compound TPDI (Figure 7) led to higher device efficiencies (measured in reverse mode, from Voc to short-circuit current density) than those for spiro-OMeTAD, although when doped, the two HTMs had similar efficiencies under 1 sun conditions.29 Undoped TPDI showed good stability for days under ambient conditions. The cathode used was a carbon electrode, which does not need high vacuum deposition, further reducing the cost of the solar cell. The combination of carbazole and diphenylamine groups has also been investigated (structures SGT-409, SGT-410, and SGT-411 in Figure 8).30 The reported efficiencies were close to those for spiro-OMeTAD, although the Voc values were below

those measured for spiro-OMeTAD. Generally, the use of a carbazole central core results in a shift to a higher HOMO energy and thus a lower Voc in the solar cells. Time-resolved photoluminescence, measured by time-correlated single photon counting (TCSPC), was used to study qualitatively the interfacial charge transfer reaction between the HTMs and the perovskite. The radiative decay becomes much faster due to efficient hole transfer from the perovskite to the HTM, as shown in Figure 8. Thiophene or polythiophene motifs are also widely used as HTMs.31 In perovskite solar cells, small molecules bearing thiophene or polythiophene motifs have also been investigated. For instance, DR3TBDTT (Figure 9)32 achieved a solar-to-energy conversion efficiency of 8% without the use of dopants but with the addition of poly-(dimethylsiloxane) (PDMS). The use of PDMS was necessary to achieve the desired hydrophobicity and film nanomorphology that led to the best devices. Yang, Sun, and co-workers33 compared the performance of two HTMs having a phenoxazine core and thiophene- and 2,2′-bithiophene-containing substituents (structures POZ2 and POZ3, respectively, in Figure 9) in organic solar cells and perovskite devices, with excellent results in both cases. The substitution of the aromatic 2,5-thiophene linker in POZ3 with the quinonoid benzo-[c][2,1,3]-thiadiazole motif in structure POZ2 improved the hole mobility (measured using SCLC). More complex molecular architectures have been synthesized for use as HTMs. For example, the perylenediimide-based HTM Th-PDI (Figure 9) reached a Voc as high as 1.23 V with a perovskite 874


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Figure 10. Examples of HTMs with conjugated (hetero)-aromatic cores.

case, the photocurrent achieved was close to 21 mA/cm2. Moreover, Nazeeruddin, Ahmad, and co-workers40,41 developed interesting HTMs with heptacyclic heteroaromatic cores (structures KR122, KR131, KR145, KR133, and HPDI in Figure 10) and demonstrated in the case of KR131 that the efficiencies matched the values reported for spiro-OMeTAD. The key novelties in the use of a heptacyclic heteroaromatic core were the introduction of electron-rich methoxy substituents and the development of an efficient four-step synthesis. The aromatic core azulene has been also incorporated into HTMs (structures HTM C, HTM D, and HTM E in Figure 11). Efficiency values as high as 16.5% at 1 sun were disclosed by Nishimura et al.42 According to the authors, the high efficiency is due to the horizontal face-on orientation of the azulene-based molecule toward the perovskite layer. Transient resolved microwave conductivity measurements supported this hypothesis.

with a larger band gap than MAPbBr3 (band gap of 2.3 eV) with almost no hysteresis in the current−voltage (I−V) curve.34 Moreover, UPS measurements again showed a difference of 0.4 eV in the HOMO energy level compared with the oxidation potential measured using cyclic voltammetry. Furthermore, structure Fused-F with a fused hexacyclic heterocyclic core (Figure 9) also achieved a very high Voc (>1 V) with a similar efficiency compared with spiro-OMeTAD using a perovskite, FAPbI3, with a narrower band gap (1.6 eV) than the above-mentioned.35 In this case, the oxidation potential measured was 5.23 eV, but no UPS measurements confirmed the HOMO energy level. Inspired by the spiro motif in spiro-OMeTAD, Wang, Tu, and co-workers36 reported a spiranic dithiophene derivative (SCPDT-BiT in Figure 9) with a Tg above 130 °C. Moreover, a Voc close to 1 V was measured. Nevertheless, the solar cell photocurrent was always low. Last but not least, the HTMs Oligomer-1, Oligomer-2, HTM A, and HTM B (Figure 10) were also designed, synthesized, and investigated in perovskite solar cells. These molecules incorporate a, S,N-heteroacene core with thiophene rings in the substituents of the core. Overall, these materials had the correct energy levels and displayed excellent Voc values close to 1 V. However, the photocurrent was sensibly lower.37,38 Ahmad and co-workers39 reported the pentacene derivative indicated in Figure 10. In this

4. PORPHYRINS AND PHTHALOCYANINES AS HTMS Porphyrins and phthalocyanines have also been designed, synthesized, and processed from solution as HTMs. Recently, porphyrins have played a major role in organic photovoltaics (OPVs) as HTMs with very low energy losses that can be compared to the best perovskite solar cells (see the structures Por-CNRod and Por-Rod in Figure 12).43 875


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Figure 11. Azulene-based HTMs.

Figure 12. Porphyrins employed as HTMs.

Figure 12). Efficiencies as high as 16.6% at 1 sun were measured.45 The reported hole mobility of YA (2.04 × 10−4 cm2 V−1 s−1) is outstanding for solution-processed porphyrin materials. Finally yet importantly, the low HOMO energy also leads to Voc values comparable to those of the standard spiro-OMeTAD.

Zhang, Gao, Huang, and co-workers early in 2016 demonstrated efficiencies of over 14% using zinc porphyrin ZnPorph (Figure 12).44 This excellent efficiency value was further increased by Chen, Yeh, and co-workers using porphyrins with 2-(4-(N,N-dialkylamino)-phenyl)-ethynyl substituents at two of the meso positions (structures YA and Y2A2 in 876


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Figure 13. Phtalocyanines employed as HTMs.

lower Voc, not only because the HOMO energy for ZnPc-(tBu)4 is higher than that for spiro-OMeTAD but also because of the faster carrier recombination of perovskite solar cells with ZnPc-(tBu)4. Furthermore, steady-state luminescence emission spectroscopy showed that the hole transfer from the perovskite to ZnPc-(tBu)4 is less efficient than that to spiro-OMeTAD. The use of an Al2O3 mesoporous scaffold has also been investigated using phthalocyanines (Sym-HTPcH in Figure 13).53 Also in this case the Voc is lower than that with spiro-OMeTAD as a result of the higher HOMO energy. Interestingly, the hysteresis in the current density−voltage (J−V) curves is almost negligible for Sym-HTPcH. The use of phthalocyanines with 2-alkylthiophene substituents (structure HT-ZnPc in Figure 13) was further explored by Torres, Nazeeruddin, and co-workers,54 and efficiencies as high as 17.5% at 1 sun were obtained. The symmetric alkyl-substituted free-base phthalocyanine C5PcH2 has been also reported (Figure 13). The charge mobility of C5PcH2 was improved upon thermal annealing at 130 °C for 10 min. This improvement was necessary to achieve efficiencies over 10% at 1 sun.55 Phthalocyanines containing Cu, Ni, and Fe as metal centers have also recently been used (structures CoPc-Cou, FePc-Cou, and NiPc-Cou in Figure 13).56 Moreover, the copper phthalocyanine CuPc-OTPA-tBu (Figure 13) led to efficiencies as high as 13.5% when oxidized with 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4TCNQ).50 Interestingly, the copper phthalocyanine TS-CuPc (Figure 13) has been also used as a co-HTM with PEDOT:PSS.57 The efficiencies were as high

More recently, the symmetrically substituted zinc porphyrin ZnP (Figure 12) with diarylaminophenyl groups at all four meso positions achieved efficiencies close to 18% at 1 sun.46 Interestingly its copper-(II) analogue CuP (Figure 12) also performed very well as an HTM. As pointed out in their work, Hua, Wong, Zhu, and co-workers assigned the lower Voc observed for CuP to an increase in the carrier recombination.46 In the case of phthalocyanines, more research has been carried out than for their porphyrin counterparts. We will focus herein only on those examples that used solution-processed phthalocyanines. Nonetheless, we acknowledge the fact that high-vacuumassisted deposition of phthalocyanines has also been reported.47−50 In particular, the different energy level offsets at the interface between the perovskite material and spiro-OMeTAD or copper-(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluoro-29H,31H-phthalocyanine (F16CuPc) measured using UPS and X-ray photoelectron spectroscopy are remarkable and should help synthetic organic chemists design future HTMs.48 Pioneering work by Torres, Nazeeruddin, and co-workers on DSSCs using efficient phthalocyanines has paved the way for useful soluble phthalocyanines as HTMs. Initial assessments with TT80 (Figure 13),51 solvents, and dopants led to efficiencies close to 6.7%. A simpler version of TT80 (i.e., compound ZnPc-(tBu)4 in Figure 13) was reported.52 The efficiency, however, was lower than with TT80, which enhances the importance of the bulky moieties that also favor better solubility. Moreover, the carrier recombination lifetime was measured using photoinduced transient photovoltage, which showed that ZnPc-(tBu)4 displays a 877


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Accounts of Chemical Research as 17.29% at 1 sun. According to the authors, the co-HTM leads to better perovskite crystal growth onto the HTM and higher carrier mobility. Moreover, the use of the copper phthalocyanine TS-CuPc leads to lower acidity on the PEDOT:PSS, thus increasing the device stability.57

molecules as hole and electron transport materials and their applications in solar cells. Lydia Cabau graduated in 2007 with a degree in chemistry from URV (Tarragona, Spain). After 3 years at Repsol as head of the chromatography unit, she moved to ICIQ to carry out her Ph.D. studies with Professor Palomares. She obtained her Ph.D. with honors in 2014 for her work on the synthesis of small molecules for DSSCs and OPVs. As a postdoctoral researcher, her work focused on the design of new hole transport materials for perovskite solar cells and characterization of the devices.

5. CONCLUSIONS AND OUTLOOK The design and synthesis of HTMs for perovskite solar cells will continue to grow. Although the dominant material for HTMs to date has been spiro-OMeTAD, we have included in this Account several examples of molecules that when processed from solution led to solar-to-energy conversion efficiencies close to 18% at 1 sun and hold promise for being more stable than spiro-OMeTAD. However, much research is needed to analyze the interface between the perovskite material and the HTM. Research in this direction is scarce, and questions about the differences in HOMO energy level alignment arise when HTMs with properties in solution identical to those of spiro-OMeTAD do not perform alike in solar cells. Moreover, studies on carrier losses due to charge recombination processes are also necessary. These reactions have to be understood in order to explain the differences in Voc for devices that a priori should deliver identical Voc. Fortunately, the increasing number of groups working on this topic ensures the rapid growth of novel materials that surely will overpass the actual record efficiencies for perovskite solar cells. The future looks exciting for the quest to replace spiroOMeTAD in perovskite solar cells, which somewhat resembles the challenge that occurred in OPVs to find and replace fullerene derivatives as electron-acceptor materials in bulk−heterojunction organic solar cells. The former has started and the latter already has some examples,58−60 demonstrating that it is feasible to find alternatives to molecules that at first glance seem to be very hard to replace in solar cells.

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Anton Vidal-Ferran graduated in 1987 with a degree in chemical engineering from IQS (Barcelona, Spain), where he completed his Ph.D. studies under Prof. P. Victory’s supervision in 1992. He then took up two postdoctoral appointments with Profs. J. K. M. Sanders (University of Cambridge, 1993−1994) and M. A. Pericàs (University of Barcelona, 1995−1999), after which he moved to Bayer-AG (Leverkusen, Germany, 1999−2003). Following his appointment as ICREA Research Professor, he started his independent career as a group leader at ICIQ (Tarragona, Spain) in 2003. His research projects aim to develop efficient asymmetric catalytic tools and functional materials. Emilio Palomares graduated in 1997 with a degree in biology from UVEG (Valencia, Spain). He moved to ITQ-UPV-CSIC (Valencia, Spain), where he completed his Ph.D. studies under Prof. Hermenegildo Garcia’s supervision in 2001. He worked as a Marie Curie Fellow with Prof. James R. Durrant at Imperial College London (2001−2004), and in 2004, as Ramon y Cajal Fellow, he moved to ICMol-UVEG (Valencia, Spain). In 2006, he moved to ICIQ (Tarragona, Spain), where he holds an ICREA Professorship. His current research focuses on energy-related molecular devices and biomolecular sensors.

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ACKNOWLEDGMENTS The authors acknowledge ICIQ-BIST (Severo Ochoa Excellence Accreditation 2014-2018 SEV-2013-0319) and ICREA. The authors are also grateful to MINECO and the Agencia Estatal de Investigación (AEI) for Projects CTQ2016-80042-R/AEI, CTQ2014-60256-P/AEI, and CTQ2017-89814-P and to the Generalitat de Catalunya for AGAUR funding. E.P. thanks Dr. Nuria Fernández-Montcada for her help.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.accounts.7b00597. Characteristics of spiro-OMeTAD as a hole transport material (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Anton Vidal-Ferran: 0000-0001-7926-1876 Emilio Palomares: 0000-0002-5092-9227 Author Contributions

All of the authors contributed equally to this work. Notes

The authors declare no competing financial interest. Biographies ́ Cristina Rodriguez-Seco received a B.S. degree in chemistry from Salamanca University (Salamanca, Spain) in 2013. She was awarded an FPI Grant by the Spanish MINECO in 2015 to carry out her Ph.D. research studies at ICIQ-BIST under the supervision of Professor Palomares. She has been focused on the synthesis of semiconductor 878


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